diff --git a/trytond/model/multivalue.py b/trytond/model/multivalue.py
index af8f55119a..a2703c054a 100644
--- a/trytond/model/multivalue.py
+++ b/trytond/model/multivalue.py
@@ -60,7 +60,7 @@ class MultiValueMixin(object):
                 return str(value)
             return value.id
         elif isinstance(value, (list, tuple)):
-            return [r.id for r in value]
+            return tuple(r.id if isinstance(r, Model) else r for r in value)
         else:
             return value